A sealed innerliner package or bag placed in a box, referred to as a bag-in-a-box packaging, is conventionally used for dry food product applications such as cereal, cracker, and snack packaging. The bag acts as a moisture barrier to keep the contents of the bag fresh and the box provides a protective outer structure for storage and handling of the package. The bag is usually heat sealed along the top, the bottom and vertically along the center of the package to form a fin seal. When the bag is formed, a moisture barrier layer is typically positioned on the outer surface of the bag and a sealant layer faces the interior of the bag. As an alternative to an innerliner package, a multilayer polymeric package, which is not stored in a box, may instead be used for dry food products. The package is optionally surface printed, such as to provide information regarding the product contained in the package.
Downgauging (i.e., reducing thickness) of packaging has been an ongoing trend to improve economics and increase sustainability. For bag-in-box applications for dry food products, key attributes for the plastic multilayer film innerliner are its moisture barrier characteristics and its abuse resistance (e.g., tear and puncture strength). A conventional multilayer film structure useful for innerliners is, for example, about 2-3 mils thick.
By using recently developed high moisture barrier high density polyethylene (HDPE) resins, such as Nova's SURPASS®, the required moisture barrier can be accomplished with a relatively thin layer of HDPE, for example between about 0.3 and 1.0 mils. However, at this thickness, a multilayer film having a conventional sealant layer lacks the necessary stiffness and abuse resistance required for an innerliner package. Even with the use of nylon in the buried layer, as is typical for many innerliner structures, the multilayer film still has poor puncture and tear properties.
One approach to downgauging of a plastic package is to machine direction orient a blown or cast film, which improves the moisture barrier capabilities and many key physical properties. However, with an HDPE based multilayer film structure typical of an innerliner package, orientation causes the film to become splitty (i.e., weak) in the machine direction. This weakness can lead to a compromise in package performance, especially during opening of the package.
In addition to an HDPE based moisture barrier, innerliner packages may also contain buried, or interior, layers of nylon or ethylene vinyl alcohol (EVOH) for oxygen protection, as well as tie, or adhesive layers to bind the outer, buried and sealant layers together. A conventional multilayer innerliner structure may comprise layers in the following order, from the outer layer to the inner layer: HDPE/tie/nylon/tie/sealant.
A drawback to the conventional multilayer innerliner structure is that the structure can tend to curl when prepared using a blown film process. During blown film processing, the buried nylon layer is the first layer to freeze up during cooling, followed by the outer HDPE layer. Moreover, HDPE goes through a large volume change as it converts from a melted resin to a solid layer. The combination of the layer cooling order and the HDPE change in volume can result in curling of the film. A film that curls significantly is difficult to process on the packaging machine.
A further drawback to the employment of a surface HDPE outside layer in an innerliner structure is that it can result in a powder build up on the customer filling equipment. This powder is primarily low molecular weight HDPE that is present on the outer HDPE layer and contributes to maintenance and Environmental Health and Safety (EHS) costs.